The objectives of this study are first to determine more accurately than has been done before the time during which normal innervation is required for complete differentiation of all components of muscle spindles. The second objective is to attempt to identify the types of neurons that have reinnervated mixed intra-and extrafusal muscle fibers that have been produced experimentally by temporary denervation. The experimental procedure for part 1 consists of crushing the nerves to the med. head of the gastrocnemius muscle in rats that are 4, 6, 7 and 8 days old. The muscles will be stained with haematein and eosin and spindles will be compared with those of the control side for deficits in number or components. The same operative procedure will be employed in newborn rats for the second part of the study with the addition of dorsal rhizotomies and partial spinal cord ablations. Staining procedures consist of reduced silver and cholinesterase techniques to identify all types of neurons which innervate the mixed fibers. These experiments are planned to add significant information to our knowledge of the specificity of the neurotrophic influence of neurons upon the development of muscle spindles.